1. Technical Field of the Invention
This invention relates generally to integrated circuits and more particularly to calibration of circuits of the integrated circuit.
2. Description of Related Art
Integrated circuits are known to be used in a wide variety of electronic equipment including radios, cellular telephones, wireless modems, home appliances, etc. One common technology for producing integrated circuits is Complimentary Metal Oxide Semiconductor, which is more commonly known as CMOS. CMOS technology has become the fabrication technology of choice for a majority of commercial grade integrated circuits due to its flexibility of design, level of integration, and cost.
While CMOS technology offers many advantages, there are some limitations. For instance, component tolerances of on-chip resistors, capacitors, and/or transistors are at best +/−5%, but more typically +/−20%. For many circuits, the component tolerance is not a critical issue, however, for precision circuit the component tolerance is a critical issue. For example, a resistor-capacitor (e.g., RC) low pass filter passes signals having frequencies below a corner frequency of the low pass filter and attenuates signals having frequencies above the corner frequency. As is known, the corner frequency is established based on the resistance value and capacitance value. With component tolerances of +/−20%, the corner frequency of a single pole RC low pass filter (i.e., a low pass filter that includes a single resistor and a single capacitor) may vary from 0.64 of the desired corner frequency when the R and C are each at the minimum component value to 1.44 of the desired corner frequency when the R and C are each at the maximum component value. This wide variation is unacceptable.
To reduce the adverse affects of the component tolerance variation, many CMOS integrated circuit (IC) designs include an RC calibration circuit and selectable resistor circuits and capacitor circuits for use in precision circuits. In general, the RC calibration circuit includes a test resistor and a test capacitor. The test resistor and test capacitor are tested, typically by applying a voltage, a pulse, and/or a ramp, to determine their actual values. The actual values are compared to the designed values to determine a difference. The difference is used to tune the selectable resistor circuits and capacitor circuits. For example, if the test resistor was designed to be a 1 Kilo-Ohm resistor and it is measured to be 900 Ohms, the actual value is 90% of the desired value. To achieve the desired value, the actual value must be increased by 1.11 (e.g., 1/0.9). Thus, the selectable resistor and capacitor circuits are adjusted by a value of 1.11.
While the RC calibration circuits allow for precision circuits to be implemented on ICs using CMOS technology, they are based on the assumption that the components of the IC have the same component offset as the test components. In many applications, this assumption is of no consequence. For highly precise circuits, such as low pass filters used in radio receivers to block local oscillation leakage, the RC calibration circuit is not sufficiently accurate.
Therefore, a need exists for a method and apparatus to calibrate highly precise IC circuits including filters.